The invention relates to the production of steel in an electric furnace. In particular it relates to methods of producing steel wherein the raw materials are comprised partly or entirely of pre-reduced iron ore and not solely of scrap.
Research is currently underway to develop siderurgical methods which can substitute for blast furnaces. The methods developed by such research seek to eliminate certain disadvantages of blast furnaces, such as the lack of flexibility in operation, and the need to employ a coke works and a facility for agglomerating the iron ore. However, a novel method in order to be deemed satisfactory should retain the excellent energy efficiency of blast furnaces. The new methods being researched are often referred to as "reduction-fusion" or "smelting reduction" processes. One of these methods utilizes a combination of two reactors, the first of which starts with iron ore and produces a treated ore which is appreciably reduced to the metal state. This treated ore is charged to a second reactor where it is transformed either into a molten iron analogous to pig iron, or directly into raw steel. The gas circuits of the two reactors may be interconnected or completely independent.
A particular embodiment of such a method is described, e.g., in an article entitled "Reduction Of Fine Ore In A Circulating Fluid Bed As The Initial Stage In Smelting Reduction" [(in English)], in a March, 1991 issue of Metallurgical Plant and Technology International, on pages 28-32, and in French Patent App. No. 91-14467 assigned to the present applicant. The reduction reaction of the iron ore is accomplished by reacting a circulating fluidized bed formed by particles of semi-coke and ore undergoing reduction, fluidized by a reducing gas mixture of CO, CO.sub.2, H.sub.2, and H.sub.2 O. The reduction reaction takes place in a first zone of the reactor. The gas and entrained particles of semi-coke and ore undergoing reduction are then passed into a cyclone separator, where the gas is withdrawn from the reactor. The particles then descend into a gasifier unit where coal and oxygen are introduced separately or together to form CO and hydrogen to contribute to the further reduction of the ore. The solid and gaseous materials are then fed back to the first zone of the reactor. Periodically or continuously a portion of the solid material (comprising reduced ore and semi-coke) in this first zone is withdrawn. Fresh ore is introduced into the stream of materials between the cyclone and the gasifier. The gas withdrawn from the cyclone is treated to remove contaminants and reintroduced into the gasifier and at the bottom of the first zone of the reactor where it can again function as a fluidizing medium and reagent.
The solid materials removed from the reactor are then sent to the fusion reactor to contribute to the production of molten metal (pig-type iron, semi-pig-type iron, or steel, depending on carbon content). As previously mentioned, these materials are comprised essentially of a mixture of substantially-reduced iron ore (reduced to the extent of at least c. 75%) and semi-coke, accompanied by minor amounts of other nonferrous materials from impurities in the ore, and possible additives.
These materials, hereinafter referred to as "pre-reduced" materials for the sake of simplicity, are distinguished from pre-reduced ores produced by other types of facilities in that the materials produced by the subject method may have particularly high levels of carbon and sulfur. The carbon content is 4-7% or more (compared to the usual 1-3%), and the sulfur content is 0.06-0.1% (compared to the usual 0.01-0.02%). The high sulfur content is caused by the use of coal as a carbon source, instead of the natural gas used in other "fusion-reduction" methods. Consequently it is desirable to carry out a desulfurization operation on the molten iron prior to converting the iron to steel, if ultimately one wishes to produce grades of steel having very low residual sulfur content (e.g. a few thousandths of a percent). Achieving such a low sulfur content solely by desulfurization of molten steel would be difficult and onerous.
The fusion reactor for the solid materials may be a converter or a steelmaking electric furnace of classical construction. An electric furnace has the advantage of lower capital and operating cost compared to a converter if the capacity of the apparatus is on the order of 100 T molten metal or less. Furthermore, an electric furnace affords great flexibility in the choice of raw materials, because the pre-reduced material can be mixed in such a furnace with any desired quantity of scrap without giving rise to problems. The scrap input may be adjusted optimally depending on the respective costs of the materials being treated, and the tolerances for residual elements (such as copper, chromium, nickel, etc.) in the final metal, which residual elements are inevitably present in scrap but only minimally present in the pre-reduced material.
Unfortunately, a classical electric furnace, because of its shape, is poorly adapted for an operation of desulfurization of the molten iron, and in general for other pretreatment operations which customarily may be called for prior to the conversion of the iron to steel (e.g. desilicization and dephosphorization). Because these operations entail intensive mixing between the metal and the slag, it is preferable to transfer the molten iron to another reactor to perform such operations; e.g., a steelmaking ladle having a truncated cone shape, or a torpedo shaped ladle of the type commonly used for transporting molten iron. After the pretreatment, the molten iron is returned to the furnace to be decarburized there. However, it then risks being polluted by the remaining molten iron which has not been pre-treated, and by the residual slag in the furnace that is rich in sulfur and phosphorus which are inevitably present following fusion. This pollution risk is aggravated if the permanent presence of a "heel" is required for steady operation of the furnace. Finally, an electric furnace installation achieves only mediocre productivity because the furnace is idle during the pre-treatment.